1. Field of the Invention
The present invention relates to liquid crystal displays (LCDs). More specifically, the present invention relates to methods and apparatus for forming alignment layers on a substrate for aligning liquid crystal molecules in multi-domain vertical alignment liquid crystal displays.
2. Discussion of Related Art
Liquid crystal displays (LCDs), which were first used for simple monochrome displays, such as calculators and digital watches, have become the dominant display technology. LCDs are used routinely in place of cathode ray tubes (CRTs) for both computer displays and television displays. Various drawbacks of LCDs have been overcome to improve the quality of LCDs. For example, active matrix displays (using thin-film transistors) replaced passive matrix displays to improve resolution, contrast ratio, viewing angle, response time and reduce ghosting.
However, the primary drawback of conventional LCDs is the viewing angle is very narrow. Even the viewing angle of active matrixes is much smaller than the viewing angle of a conventional cathode ray tube (CRT) display. Specifically, a viewer directly in front of an LCD receives a high quality image; however viewers to the side of the LCD would not receive a high quality image. Multi-domain vertical alignment liquid crystal displays (MVALCDs) were developed to improve the viewing angle of LCDs. However, the primary drawback of MVA LCDs is the high cost of manufacturing LCDs. FIGS. 1(a)-1(b) illustrate the basic structure and functionality of a pixel of a multi Multi-domain vertical alignment liquid crystal displays (MVALCDs) 100. For clarity, MVALCD 100 of FIGS. 1(a)-1(b) is described for grayscale operation.
MVALCD 100 has a first polarizer 105, a first substrate 110, a first electrode 120, a first alignment layer 125, liquid crystals 135, liquid crystals 137, a second alignment layer 140, a second electrode 145, a second substrate 150, a second polarizer 155, and protrusions 160. Alignment layers 125 and 140 are typically formed using a polyimide (PI) film coating. A light source (not shown) sends light from beneath first polarizer 105, which is attached to first substrate 110. The polarization of the first polarizer 105 is generally directed in a first direction and the polarization of the second polarizer 155 is directed perpendicularly to first polarizer 105. Thus, light from the light source would not pass through both first polarizer 105 and second polarizer 155 unless the polarization of the light were to be rotated by 90 degrees between first polarizer 105 and second polarizer 155. For clarity, very few liquid crystals are shown. In actual displays, liquid crystals are rod like molecules, which are approximately 5 angstroms in diameter and 20-25 angstroms in length. Thus, there are over 10 million liquid crystal molecules in a pixel that is 100 μm width by 300 μm length by 3 μm height.
In FIG. 1(a), liquid crystals 135 and 137 are vertically aligned. Specifically, alignment layers 125 and 140 align the liquid crystals in the desired resting position. In the vertical alignment, liquid crystals 135 and 137 would not rotate the polarization of the light from the light source. Thus, light from the light source would not pass through LCD 100. However, as illustrated in FIG. 1(b), when an electric voltage is applied between first electrode 120 and second electrode 140, liquid crystals 135 and 137 reorientate to a tilted position. Specifically, liquid crystals 135 tilt to the left to form a first domain while liquid crystals 137 tilt to the right to form a second domain due to protrusions 160. Liquid crystals in the tilted position rotate the polarization of the polarized light coming through first polarizer 105 by ninety degrees so that the light can then pass through second polarizer 155. The amount of tilting, which controls the rotation of the polarization of the light and thus the amount of light passing through the LCD (i.e., brightness of the pixel), is proportional to the applied voltage of the electric field. Having multiple domains (i.e. liquid crystals 135 and liquid crystals 137) increases the viewing angle of the MVALCD. Generally, a single thin-film-transistor (TFT) is used for each pixel. However for color displays, a color pixel is divided into 3 color components and a separate TFT is used for each color component (typically, Red, Green, and Blue)
The primary drawback of MVA LCDs is the high cost of manufacturing LCDs. While the material cost of polyimide is very low, the process to form of alignment layers 122 and 138 is very costly. Reducing the fabrication cost of the alignment layer can greatly reduce the overall cost of manufacturing liquid crystal displays. Hence there is a need for low cost processes and apparatuses to produce alignment layers in MVA liquid crystal displays.